This invention relates to a process for recycling amido-carboxylic acid esters into amido-carboxylic acids. More particularly, the invention relates to the recycling of waste materials that are produced during the preparation of a phenyl ester salt bleach activators. These waste materials typically contain amido-carboxylic acid esters which, through the invention, may be converted into amido-carboxylic acids. These amido-carboxylic acids may then be recycled and used to prepare the phenyl ester bleach activators.
Hypochlorite and hydrogen peroxide are well known for their bleaching properties. As a bleaching agent in laundry detergents, hydrogen peroxide has the advantage of being safe to use with many fabric dyes. However, hydrogen peroxide bleaches are not effective at temperatures below 50xc2x0 C. This limits their use as most laundering is carried out at temperatures below about 40xc2x0 C. For this reason, various peroxyacids were developed as alternative bleaching agents for use in laundry detergents. The peroxyacids were generally found to be effective bleaching agents at the lower laundering temperatures. Because of their chemical instability and potential safety hazards, however, peroxyacids themselves are generally unsuitable for storage and handling.
Bleach activators were developed to address storage and handling concerns associated with peroxyacids. Bleach activators have the ability to hydrolyze under laundering conditions, effectively producing peroxyacids, even at lower temperatures, e.g. below 40xc2x0 C. By perhydrolyzing bleach molecules, bleach activators enhance the activity, and thus the cleaning ability, of a laundry detergent. Bleach activators have the further advantage of being stable when stored in solid form at room temperature. These properties permit the use of bleach activators in a wide variety of laundry detergents and other cleaning formulations.
An important class of bleach activators is phenyl ester salts. An effective bleach activator, phenyl ester salts readily react with bleach to form the corresponding peroxyacid. Exemplary phenyl ester salts, which are used as bleach activators, are described in U.S. Pat. Nos. 4,634,551; 4,852,989; 5,391,780; 5,393,905; 5,393,901; 5,414,099; 5,466,840; 5,523,434; 5,650,527; and 5,717,118; as well as in published PCT applications WO 94/18159, WO 95/07883, WO 96/16148, and WO 99/09004. These U.S. Patents and published PCT applications are incorporated herein in their entirety.
Generally, phenyl ester salts are compounds of formula (I) or (II): 
In formula (I) or (II), R1 is selected from C1-C22 alkyl, C2-C22 alkenyl, C2-C22 alkynyl, C3-C22 cycloalkyl, and C6-C14 aryl. Preferably, R1 is selected from C6-C10 alkyl, and C6-C10 aryl.
R2 and R5 are each independently selected from hydrogen, C1-C22 alkyl, C2-C22 alkenyl, C2-C22 alkynyl, C3-C22 cycloalkyl, and C6-C14 aryl. Alternatively, in formula II, R2 and R5, together with the nitrogen carrying them, form a C3-C10 heterocycle. This heterocycle may or may not contain additional heteroatoms selected from: nitrogen, oxygen, sulfur, or phosphorous. Preferably, R2 is hydrogen, and R5 is selected from hydrogen, C6-C10 alkyl, and C6-C10 aryl.
R3 and R4 are each independently selected in each instance from hydrogen, C1-C10 alkyl, C2-C10 alkenyl, C2-C10 alkynyl, C3-C10 cycloalkyl, and C6-C10 aryl. The R3 and R4 groups, together with the carbon carrying them, may form a C3-C10 cycloalkyl group. This cycloalkyl group may or may not contain heteroatoms selected from: nitrogen, oxygen, sulfur, or phosphorous. Preferably, R3 and R4 are independently selected in each instance from hydrogen and methyl.
The substituent Y on the phenyl ring is selected from SO3xe2x88x92M+, CO2xe2x88x92M+, SO4xe2x88x92M+, and N+(R7)3Xxe2x88x92. M represents a cation, and may be selected from hydrogen, ammonium and alkali metal atom. R7 in each instance is independently a C1-C4 alkyl group. X is an anion, and may be selected from a halide, hydroxide, methylsulfate, or acetate ion. Preferably, Y is selected from SO3xe2x88x92M+, and CO2xe2x88x92M+; where M is a sodium ion.
The phenyl ring may also be further substituted with 1 to 4 other substituents. The substituents may be electron-withdrawing or electron-donating groups. The substituents may, for example, be chosen to adjust the perhydrolysis rate, to adjust the hydrophilic/hydrophobic nature of the phenyl ester salt, or to adjust the solubility of the phenyl ester salt. Possible groups include, but are not limited to, hydroxyl, halogen, C1-C10 alkyl, C2-C10 alkenyl, C2-C10 alkynyl, C3-C10 cycloalkyl, and C1-C10 alkoxy, and amino groups. When the phenyl ester salts are to be used as bleach activators, it is also desirable to have electron-withdrawing groups on the phenyl ring, to facilitate perhydrolysis or bleach activation.
The value of xe2x80x9cmxe2x80x9d is 0 or 1 and represents the presence or absence of an amido group in the compound. Acceptable values for xe2x80x9cnxe2x80x9d may range from about 0 to 20, and preferred values for xe2x80x9cnxe2x80x9d include from about 0 to about 6.
Phenyl ester salts can be prepared in various ways. For example, sodium nonanamidohexanoyloxybenzenesulfonate can be prepared by reacting a C9 fatty acid with caprolactam to form 6-nonanoylamidohexanoic acid. Sodium p-hydroxybenzenesulfonate and acetic anhydride is then reacted with the 6-nonanoylamidohexanoic acid in a solvent to form sodium nonanamidohexanoyloxybenzenesulfonate. This reaction scheme is shown below in Scheme 1: 
Notable by-products from the synthesis of phenyl ester salts include various polyamido acids (nonanamido[hexanamido]-n-hexanoic acid), sodium nonamidohexanolyloxybenzenesulfonate, and sodium nonanoyloxybenzenesulfonate. Purification of phenyl ester salts, such as by solvent crystallization, produces a waste stream containing a substantial amount of amido esters. Further waste is created when a substandard phenyl ester salt is produced and cannot be economically purified further. Although U.S. Pat. No. 5,393,901 discloses a method of maximizing recovery and recycling reactants in a process for preparing amido-carboxylic acids, it does not disclose any methods of recycling these waste materials.
Recycling such waste material into useful materials and ultimately into a purified phenyl ester salt would reduce both the waste disposal costs and costs associated with the overall preparation of phenyl ester salts and detergent formulations. Accordingly, there exists a need to recycle waste materials containing amido-carboxylic acid esters into amido-carboxylic acids that may be used to prepare phenyl ester salts.
In view of the industry""s need to recycle waste materials containing amido-carboxylic acid esters waste, the invention offers valuable processes for recycling such waste. Accordingly, the invention relates to a process for recycling amido-carboxylic acid esters into amido-carboxylic acids. The process combines a waste material containing amido-carboxylic acid esters with an aqueous solution of a strong acid having a pH  less than about 1 to form a mixture. The process heats the mixture to a temperature and for a time sufficient to hydrolyze the amido-carboxylic acid esters to amido-carboxylic acids. Sufficient base is added to adjust the pH of the mixture to  greater than about 2. The mixture is then maintained at a temperature below reflux for a time sufficient to allow phase separation of the liquid aqueous and organic phases within the mixture. The organic phase is then separated from the aqueous phase and the amido-carboxylic acids recovered from the organic phase.